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Abstract:

A concentration detecting apparatus that detects the alcohol
concentration of an alcohol blended fuel includes frequency controlling
means that controls the frequency of an alternating-current voltage
applied between a pair of electrodes spaced apart from each other. A
first resistance constituent value between the electrodes is detected by
applying an alternating-current voltage at a first frequency at which a
capacitance constituent value of the impedance is zero. Similarly, a
second resistance constituent value between said electrodes is detected
by applying an alternating-current voltage at a second frequency
different from the first frequency at which the capacitance constituent
value of the impedance is zero. The alcohol concentration is calculated
based on the difference between the first resistance constituent value
and the second resistance constituent value.

Claims:

1. A concentration detecting apparatus that detects an alcohol
concentration of an alcohol blended fuel, comprising: a frequency
controlling device that controls a frequency of an alternating-current
voltage applied between a pair of electrodes spaced apart from each
other; a resistance constituent value detecting device that detects a
first resistance constituent value between the electrodes in a case where
an alternating-current voltage is applied at a first frequency at which a
capacitance constituent value of an impedance is zero and detects a
second resistance constituent value between the electrodes in a case
where an alternating-current voltage is applied at a second frequency
different from the first frequency at which the capacitance constituent
value of the impedance is zero; and a concentration estimating device
that estimates the alcohol concentration based on the difference between
the first resistance constituent value and the second resistance
constituent value.

2. The concentration detecting apparatus according to claim 1, further
comprising: a temperature detecting device that detects a temperature of
the alcohol blended fuel, wherein the concentration estimating device
estimates the alcohol concentration based on the difference between the
first resistance constituent value and the second resistance constituent
value and the temperature detected by the temperature detecting device.

3. The concentration detecting apparatus according to claim 1, further
comprising: a capacitance constituent value calculating device that
calculates the capacitance constituent value between the electrodes in a
case where an alternating-current voltage is applied at a predetermined
third frequency between the first frequency and the second frequency; and
a temperature estimating device that determines a temperature of the
alcohol blended fuel based on the difference between the first resistance
constituent value and the second resistance constituent value and the
capacitance constituent value.

4. The concentration detecting apparatus according to claim 1, further
comprising: a temperature detecting device that detects the temperature
of the alcohol blended fuel; a capacitance constituent value calculating
device that calculates the capacitance constituent value between the
electrodes in a case where an alternating-current voltage is applied at a
predetermined third frequency between the first frequency and the second
frequency; and a water concentration calculating device that calculates a
water concentration of the alcohol blended fuel based on the difference
between the first resistance constituent value and the second resistance
constituent value, the capacitance constituent value and the temperature
detected by the temperature detecting device.

5. The concentration detecting apparatus according to claim 2, further
comprising: a temperature detecting device that detects the temperature
of the alcohol blended fuel; a capacitance constituent value calculating
device that calculates the capacitance constituent value between the
electrodes in a case where an alternating-current voltage is applied at a
predetermine third frequency between the first frequency and the second
frequency; and a water concentration calculating device that calculates a
water concentration of the alcohol blended fuel based on the difference
between the first resistance constituent value and the second resistance
constituent value, the capacitance constituent value and the temperature
detected by the temperature detecting device.

6. A concentration detecting apparatus that detects an alcohol
concentration of an alcohol blended fuel by: controlling a frequency of
an alternating-current voltage applied between a pair of electrodes
spaced apart from each other; detecting a first resistance constituent
value between the electrodes in a case where an alternating-current
voltage is applied at a first frequency at which a capacitance
constituent value of an impedance is zero; detecting a second resistance
constituent value between the electrodes in a case where an
alternating-current voltage is applied at a second frequency different
from the first frequency at which the capacitance constituent value of
the impedance is zero; and estimating the alcohol concentration based on
the difference between the first resistance constituent value and the
second resistance constituent value.

Description:

TECHNICAL FIELD

[0001] The present invention relates to a concentration detecting
apparatus. More specifically, it relates to a concentration detecting
apparatus suitable for detection of the alcohol concentration of an
alcohol blended fuel supplied to an internal combustion engine.

BACKGROUND ART

[0002] From the view point of reducing gasoline consumption, alcohol,
which emits smaller amounts of CO and HC, has recently been attracting
attention as a fuel for internal combustion engines. A known example is a
flexible-fuel vehicle (FFV) provided with an internal combustion engine
capable of running on an alcohol blended gasoline. Fuel mixtures
containing alcohol have different optimum air-fuel ratios depending on
the alcohol concentration. Therefore, to properly control the air-fuel
ratio, there is a demand for a simple apparatus that can more accurately
grasp the alcohol concentration of the fuel mixture.

[0003] Patent Literature 1 discloses a conventional concentration
detecting apparatus that detects the alcohol concentration of an alcohol
blended fuel. In the concentration detecting apparatus according to
Patent Literature 1, an alcohol concentration sensor and a coil L are
connected in series with each other. The conductivity of the alcohol
concentration sensor is detected by applying a low current to the
circuit. In addition, the resonance frequency of the LC resonant circuit
formed by the alcohol concentration sensor and the coil L is detected as
an equivalent capacitance value. The frequency is converted into a
voltage value by a frequency-voltage conversion calculation, thereby
calculating the capacitance of the alcohol concentration sensor. The
concentration detecting apparatus according to Patent Literature 1
determines the alcohol concentration of the fuel mixture based the
capacitance.

CITATION LIST

Patent Literature

Patent Literature 1: Japanese Utility Model Laid-Open No. 5-33054

Patent Literature 2: Japanese Patent Laid-Open No. 7-306172

Patent Literature 3: Japanese Patent Laid-Open No. 2009-145131

SUMMARY OF THE INVENTION

Problem to be Solved by the Invention

[0004] In the conventional concentration detecting apparatus such as
disclosed in Patent Literature 1, the conductivity and the capacitance
are detected by detecting the voltage and the resonance frequency in the
entire circuit including the alcohol concentration sensor and the
capacitor, the coil and the like connected to the sensor. Therefore, the
detection values include not only a constituent value attributed to the
fuel but also constituent values attributed to other factors, such as the
electrode of the sensor, the capacitor, the coil, the lead and the like
(referred to collectively as the electrode and the like hereinafter).
Therefore, variations of the conductivity and the capacitance are
influenced not only by the alcohol concentration but also by
deterioration of the electrode and the like. Therefore, the more
remarkable the deterioration of the electrode and the like, the more
remarkable the influence of the deterioration on the variations of the
conductivity and the capacitance can be, and the more remarkable the
deviation of the calculated alcohol concentration from the actual
concentration can be.

[0005] In view of such circumstances, an object of the present invention
is to solve the problem described above and provide an improved
concentration detecting apparatus that can detect an alcohol
concentration while reducing error of the detection value due to
deterioration of an electrode or the like.

Means for Solving the Problem

[0006] In accomplishing the above object, the first invention is of a
concentration detecting apparatus that detects an alcohol concentration
of an alcohol blended fuel, the concentration detecting apparatus
comprising:

[0007] frequency controlling means that controls a frequency of an
alternating-current voltage applied between a pair of electrodes spaced
apart from each other;

[0008] resistance constituent value detecting means that detects a first
resistance constituent value between the electrodes in a case where an
alternating-current voltage is applied at a first frequency at which a
capacitance constituent value of an impedance is zero and detects a
second resistance constituent value between the electrodes in a case
where an alternating-current voltage is applied at a second frequency
different from the first frequency at which the capacitance constituent
value of the impedance is zero; and

[0009] concentration estimating means that estimates the alcohol
concentration based on the difference between the first resistance
constituent value and the second resistance constituent value.

[0010] The second invention, according to the first invention, further
comprises temperature detecting means that detects a temperature of the
alcohol blended fuel,

[0011] wherein the concentration estimating means estimates the alcohol
concentration based on the difference between the first resistance
constituent value and the second resistance constituent value and the
temperature detected by the temperature detecting means.

[0012] The third invention, according to the first invention, further
comprises:

[0013] capacitance constituent value calculating means that calculates the
capacitance constituent value between the electrodes in a case where a
predetermined third frequency between the first frequency and the second
frequency is applied; and

[0014] temperature estimating means that determines a temperature of the
alcohol blended fuel based on the difference between the first resistance
constituent value and the second resistance constituent value and the
capacitance constituent value.

[0015] The fourth invention, according to the first or the second
invention, further comprises:

[0016] a temperature detecting means that detects the temperature of the
alcohol blended fuel;

[0017] capacitance constituent value calculating means that calculates the
capacitance constituent value between the electrodes in a case where a
third frequency between the first frequency and the second frequency is
applied; and

[0018] water concentration calculating means that calculates a water
concentration of the alcohol blended fuel based on the difference between
the first resistance constituent value and the second resistance
constituent value, the capacitance constituent value and the temperature
detected by the temperature detecting means.

Effects of the Invention

[0019] According to a first aspect of the present invention, the alcohol
concentration is determined based on the difference between the first
resistance constituent value and the second resistance constituent value
that are associated with the alternating-current voltages at different
first and second frequencies at both of which the capacitance constituent
value of the impedance is zero. Therefore, the resistance constituent
values attributed to the electrode, the leads or the like in the
concentration detecting apparatus can be removed from the resistance of
the entire circuit of the apparatus. As a result, the influence of the
deterioration or the like of the electrodes or the like on the detection
value can be removed, and the alcohol concentration can be accurately
determined based only on the resistance constituent value attributed to
the fuel.

[0020] According to a second aspect of the present invention, the alcohol
concentration is estimated based on the difference between the first
resistance constituent value and the second resistance constituent value
and the temperature of the fuel mixture. Since the conductivity of the
alcohol varies with the temperature, the alcohol concentration can be
more accurately estimated by taking the temperature into consideration.

[0021] According to a third aspect of the present invention, not only the
first resistance constituent value and the second resistance constituent
value but also the capacitance constituent value associated with the
predetermined third frequency is calculated. Both the resistance
constituent value and the capacitance constituent value have correlations
with the temperature. Therefore, not only the alcohol concentration but
also the temperature can be detected by detecting the difference between
the first resistance constituent value and the second resistance
constituent value and the capacitance constituent value. As a result,
there is no need for additionally installing the temperature sensor or
the like, and the cost of the system can be reduced.

[0022] According to a fourth aspect of the present invention, the alcohol
concentration and the water concentration of the fuel mixture can be
detected by using the difference between the first resistance constituent
value and the second resistance constituent value, the capacitance
constituent value and the temperature as parameters. Therefore, the
properties of the fuel can be more accurately detected, and the air-fuel
ratio can be more precisely controlled.

BRIEF DESCRIPTION OF DRAWINGS

[0023] FIG. 1 is a schematic diagram for illustrating entirely
arrangements of a system according to an embodiment 1 of the present
invention.

[0024] FIG. 2 is an equivalent circuit diagram of a concentration
detecting apparatus 2 according to the embodiment 1 of the present
invention.

[0025] FIG. 3 is a graph for illustrating variations of the resistance of
a metal electrode and a conductive material with temperature.

[0026] FIG. 4 is a complex impedance plot showing a variation of the
impedance when an alternating-current frequency is applied to the
detecting circuit of the concentration detecting apparatus according to
the embodiment 1 of the present invention.

[0027] FIG. 5 is a flowchart for illustrating a control routine performed
by the controller in the embodiment 1 of the present invention.

[0028] FIG. 6 is a graph for illustrating a relationship among
conductivity, capacitance and temperature of the concentration detecting
apparatus according to an embodiment 2 of the present invention.

[0029] FIG. 7 is a flowchart for illustrating a control routine preformed
by the controller in the embodiment 2 of the present invention.

[0030] FIG. 8 is a graph for illustrating a relationship between
equivalent concentration value and water content of the concentration
detecting apparatus according to an embodiment 3 of the present
invention.

MODES FOR CARRYING OUT THE INVENTION

[0031] In the following, embodiments of the present invention will be
described with reference to the drawings. In the drawings, the same or
equivalent components are denoted by the same reference numerals, and
descriptions thereof will be simplified or omitted.

Embodiment 1

[0032] FIG. 1 is a schematic diagram for illustrating a state of
installation of a concentration detecting apparatus according to an
embodiment 1 of the present invention. As shown in FIG. 1, a
concentration detecting apparatus 2 is used to detect the alcohol
concentration of a fuel mixture, such as an alcohol blended gasoline.
FIG. 1 shows an example in which the concentration detecting apparatus is
mounted on a fuel path 6 or the like of an internal combustion engine 4
mounted on a vehicle or the like. However, the site of installation or
use of the concentration detecting apparatus 2 according to the present
invention is not limited to this site, and the concentration detecting
apparatus 2 can be used at any site as required for detection of the fuel
concentration.

[0033] The concentration detecting apparatus 2 has a pair of electrodes 8
spaced apart from each other. The electrodes 8 are disposed in the fuel
path 6 with at least part thereof being in contact with the fuel mixture.
The concentration detecting apparatus 2 has an alternating-current power
supply 10 connected to the electrodes 8 to apply an alternating-current
or direct-current voltage to the electrodes 8. Although not shown in the
drawing, the concentration detecting apparatus 2 forms a detecting
circuit to which an impedance detector that detects the impedance between
the electrodes 8, a frequency detector that detects the
alternating-current frequency and the like are connected.

[0034] The concentration detecting apparatus 2 further has a controller
12. The controller 12 is connected to various detectors and the
alternating-current power supply 10 in the concentration detecting
apparatus 2. The controller 12 receives output signals of the detectors,
detects the impedance or the like of the concentration detecting
apparatus 2, and performs various kinds of calculations based on the
information obtained by the detection. In addition, for example, the
controller 12 issues a control signal to the alternating-current power
supply 10 to control the frequency of the voltage applied to the
concentration detecting apparatus 2 or the like.

[0035] Gasoline and alcohol in the fuel mixture have significantly
different conductivities and dielectric constants. Alcohol has higher
conductivity and dielectric constant than gasoline. Therefore, the
dielectric constant and the conductivity of the fuel mixture vary more
remarkably with the alcohol concentration. Therefore, the alcohol
concentration of the fuel mixture can be detected by detecting the
resistance value or capacitance between the electrodes 8.

[0036] The impedance that occurs when an alternating-current voltage is
applied to the concentration detecting apparatus 2 can be broken down
into the following constituents:

[0037] (1) constituents attributed to the fuel between the electrodes 8;
and

[0038] (2) constituents attributed to the other factors than the fuel such
as the electrodes 8.

[0039] Note that the capacitance constituents attributed to the other
factors than the fuel, such as the electrodes 8, classified above as the
category (2) are canceled by a capacitor inserted in the sensor circuit,
for example, and therefore are negligible in this example. Therefore, the
concentration detecting apparatus 2 has a configuration shown by the
equivalent circuit diagram of FIG. 2.

[0040] FIG. 2 is an equivalent circuit diagram showing the concentration
detecting apparatus 2 according to the embodiment 1 of the present
invention. In the equivalent circuit diagram of FIG. 2, a fuel resistance
constituent Rf and a fuel-derived capacitance constituent Cf are
constituents attributed to the fuel mixture between the electrodes 8
classified above as the category (1), and the electrode resistance
constituent Re is a constituent attributed to other factors than the
fuel, such as the electrodes 8, classified above as the category (2).

[0041] In the equivalent circuit diagram, the fuel resistance constituent
Rf and the fuel-derived capacitance constituent Cf are the constituents
that vary with the alcohol concentration of the fuel mixture. Therefore,
a variation of the alcohol concentration can be detected by detecting a
variation of the fuel resistance constituent Rf.

[0042] However, the value of the resistance value detected when an
alternating-current or direct-current voltage is applied to the entire
circuit includes the electrode resistance constituent Re classified above
as the category (2). If the electrode resistance constituent Re were
fixed, a variation of the fuel resistance constituent Rf could be easily
singly detected. However, the electrode resistance constituent Re varies
with deterioration of the electrodes or with temperature.

[0043] FIG. 3 is a graph for illustrating variations of the resistance of
a metal electrode and a conductive material with temperature, in which
the horizontal axis indicates the temperature, and the vertical axis
indicates the resistance. In FIG. 3, the dashed line (a) indicates the
resistance of the metal electrode, and the curve (b) indicates the
variation of the resistance of the conductive material.

[0044] As shown in FIG. 3, the resistance of the metal electrode increases
with the temperature rises. On the other hand, the resistance of the
conductive material decreases as the temperature rises. This shows that
in the concentration detecting apparatus 2, the electrode resistance
constituent Re, which is a resistance constituent attributed to the
electrodes 8 or the like, increases as the temperature rises, and the
fuel resistance constituent Rf attributed to the fuel mixture, which is a
conductive material, decreases as the temperature rises.

[0045] As described above, the electrode resistance constituent Re and the
fuel resistance constituent Rf vary with temperature in the opposite
ways. To accurately detect the variation of the resistance attributed to
the alcohol concentration of the fuel mixture, the variations of the
resistance of the electrode resistance constituent Re and the fuel
resistance constituent Rf with temperature that occur in the opposite
ways have to be removed before the variation of the fuel resistance
constituent Rf with alcohol concentration is measured.

[0046] The resistance value of the leads or the like forming the
electrodes 8 or the detecting circuit of the concentration detecting
apparatus 2 varies as the leads or the like deteriorate with time. In
particular, the electrodes 8 are disposed in the fuel mixture and
therefore can significantly deteriorate and significantly vary in
resistance. Therefore, to accurately detect the variation of the
resistance value with the alcohol concentration, it is also important to
remove the variation of the electrode resistance constituent Re
attributed to the deterioration of the electrodes 8.

[0047] In view of the above description, according to the embodiment 1, an
alternating-current voltage is applied to the circuit of the
concentration detecting apparatus 2 so that the electrode resistance
constituent Re and the fuel resistance constituent Rf can be separately
detected as described below. FIG. 4 is a complex impedance plot showing a
variation of the impedance of the concentration detecting apparatus 2
detected when an alternating-current voltage is applied to the detecting
circuit of the concentration detecting apparatus 2 while sweeping
(changing) the frequency of the alternating-current voltage. In FIG. 4,
the horizontal axis indicates a real-number part (resistance constituent)
of the impedance, and the vertical axis indicates an imaginary-number
part (capacitance constituent).

[0048] As shown in FIG. 4, when an alternating-current voltage is applied
to the circuit of the concentration detecting apparatus 2, the
constituents attributed to the fuel classified above as the category (1)
and the constituents attributed to the other factors (electrodes or the
like) than the fuel classified above as the category (2) can be
separately detected because of the difference in physical properties
therebetween.

[0049] In FIG. 4, a resistance value R1 (first resistance constituent
value) indicated by an intersection of the curve indicating the complex
impedance and the x axis is the electrode resistance constituent Re. On
the other hand, a resistance value R2 (second resistance constituent
value) indicated by another intersection is a sum of the electrode
resistance constituent Re and the fuel resistance constituent Rf.
Therefore, if the resistance values R1 and R2 are detected, the fuel
resistance constituent Rf can be determined according to Rf=R2-R1.

[0050] A first frequency f1 and a second frequency f2 corresponding to the
resistance values R1 and R2 are fitted values that can be determined if
the composition of the fuel mixture, the temperature range in which the
fuel mixture is used or the like is identified to some extent. Therefore,
in this embodiment 1, the first frequency f1 and the second frequency f2
are set by experiment or other means at appropriate values according to
the composition or use environment of the fuel mixture and previously
stored in the controller 12. In concentration detection, the first and
second frequencies f1 and f2 previously stored are applied to detect the
impedances, thereby detecting the resistance values R1 and R2.

[0051] In the embodiment 1, considering that the fuel mixture is an
alcohol blended gasoline, for example, the first frequency f1 is set at a
frequency of 10 [kHz] to 1 [MHz], and the second frequency f2 is set at a
frequency of 100 [Hz] to 10 [kHz].

[0052] As can be seen from the above description, the fuel resistance
constituent Rf determined from the resistance values R1 and R2 is
considered as a resistance that includes no resistance attributed to the
electrodes of the sensor and the like and is attributed only to the fuel.
The fuel resistance constituent Rf has a correlation not only with the
alcohol concentration but also with the temperature. Therefore, in this
embodiment 1, the relations of the fuel resistance constituent Rf with
the alcohol concentration and the temperature are previously found and
stored in the controller 12 as a map. In concentration detection, the
alcohol concentration is calculated based on the map using the fuel
resistance constituent Rf and the temperature T of the fuel mixture
determined from the output of the temperature sensor or the like as
parameters.

[0053] FIG. 5 is a flowchart for illustrating a control routine performed
by the controller in the embodiment 1 of the present invention. The
routine shown in FIG. 5 is a routine that is repeatedly performed at
regular intervals during operation of the internal combustion engine 4.
According to the routine shown in FIG. 5, whether the internal combustion
engine 4 has been started or not is first detected (S12). If the internal
combustion engine 4 is out of service, detection of the fuel
concentration is unnecessary, so that the routine ends.

[0054] However, if it is recognized that the internal combustion engine 4
has been started, it is then determined whether the concentration
detecting apparatus 2 is in the normal state or not (S14). For example,
if the concentration detecting apparatus 2 has not yet been warmed up to
an operating temperature, it is not recognized that the concentration
detecting apparatus 2 is in the normal state. If it is not recognized
that the concentration detecting apparatus 2 is in the normal state, the
routine ends.

[0055] However, if it is recognized in Step S14 that the concentration
detecting apparatus 2 is in the normal state, the temperature T is then
detected (S16). The temperature T is detected by the controller 12 in
response to an output signal from a temperature sensor (not shown)
disposed in the fuel path 6.

[0056] Then, an alternating-current voltage at the first frequency is
applied to the circuit of the concentration detecting apparatus 2 to
detect the impedance (S18). More specifically, the first frequency f1
previously stored in the controller 12 is read out, and the controller 12
outputs a predetermined control signal to the alternating-current power
supply 10 to apply the alternating-current voltage at the first frequency
f1 between the electrodes 8. Then, the resulting impedance is detected.

[0057] Then, an alternating-current voltage at the second frequency f2 is
applied to the circuit of the concentration detecting apparatus 2 to
detect the impedance (S20). More specifically, the second frequency f2
previously stored in the controller 12 is read out. Then, the controller
12 outputs a predetermined control signal to the alternating-current
power supply 10 to apply the alternating-current voltage at the second
frequency f2 between the electrodes 8, and the resulting impedance is
detected.

[0058] Then, based on the impedances detected in Steps S18 and S20, the
fuel resistance constituent Rf is determined (S22). The fuel resistance
constituent Rf is the difference between the resistance value R1
determined from the impedance associated with the first frequency and the
resistance value R2 determined from the resistance constituent associated
with the second frequency and is determined according to the formula: the
fuel resistance constituent Rf=resistance value R2-resistance value R1.

[0059] Then, based on the fuel resistance constituent Rf and the current
temperature T, the alcohol concentration is calculated (S24). The alcohol
concentration is determined according to the map that indicating the
relationship among the temperature T, the fuel resistance constituent Rf
and the alcohol concentration. The map is previously stored in the
controller 12. Then, the routine ends.

[0060] As described above, according to the embodiment 1, the electrode
resistance constituent Re attributed to the electrodes 8 and the like and
the fuel resistance constituent Rf attributed to the fuel can be
separately detected. The alcohol concentration of the fuel mixture can be
detected relying only on the variation of the fuel resistance constituent
Rf attributed to the fuel by removing the influence of the variation of
the resistance value due to deterioration or temperature variation of the
electrodes 8 or the like. Therefore, the alcohol concentration can be
more accurately detected.

[0061] In the embodiment 1, a case has been described where the resistance
values R1 and R2 are determined from the impedances in the cases where
the alternating-current voltages at the first and second frequencies f1
and f2 are applied. However, the present invention is not limited to this
implementation. For example, the resistance values R1 and R2 may be
determined according to an alternating-current impedance method by
performing a plurality of concentration detections by sweeping the
frequency from a lower frequency to a higher frequency in each
concentration detection.

[0062] Besides, in the case described above, the temperature of the fuel
mixture is detected, and the fuel concentration is determined from the
temperature and the value of the fuel resistance constituent Rf. However,
the present invention is not limited to this implementation. If the
variation of the value of the fuel resistance constituent Rf with the
temperature is negligible, the fuel concentration can be determined from
only the value of the fuel resistance constituent Rf.

[0064] A concentration detecting apparatus according to an embodiment 2
has the same configuration as the apparatus shown in FIG. 1. The
concentration detecting apparatus according to the embodiment 2 differs
from the apparatus according to the embodiment 1 only in that the
temperature of the fuel mixture as well as the alcohol concentration is
detected.

[0065] FIG. 6 is a graph for illustrating a relationship between the
conductivity (the inverse of the resistance value) and the capacitance of
the fuel. As described above, the fuel resistance constituent has a
correlation with the temperature. As shown in FIG. 6, the capacitance of
the fuel also has a correlation with the temperature and varies with the
temperature. More specifically, the conductivity increases as the
temperature rises, whereas the capacitance decreases as the temperature
rises. In addition, the conductivity has a correlation with the alcohol
concentration as described above. Therefore, the alcohol concentration
and the temperature can be detected at the same time by using the
conductivity and the capacitance as parameters.

[0066] The controller 12 stores the relationship among the conductivity,
the capacitance and the temperature shown in FIG. 6 in the form of a map.
The fuel concentration and the temperature can be detected at the same
time by detecting the conductivity (resistance value) and the
capacitance.

[0067] The value of the capacitance of the fuel mixture is taken when the
fuel-derived capacitance constituent Cf is at the maximum in FIG. 4
described earlier. The frequency and the value of the resistance
constituent at the time when the fuel-derived capacitance constituent Cf
is at the maximum are referred to as a third frequency f3 and a
resistance value R3, respectively. Then, the relationship expressed by
the following formula (1) holds.

R3Cf=1/(2πf3) (1)

[0068] The resistance value R3 can be approximately considered to be an
intermediate value between the resistance value R1 and the resistance
value R2 in FIG. 4 and is assumed in this example to satisfy the
relation: R3=R1+R2/2. The third frequency f3 at the time when the
resistance value is the resistance value R3 is previously identified. As
with the first and second frequencies f1 and f2, the third frequency f3
is a fitted value that can be determined if the composition of the fuel
mixture, the temperature range in which the fuel mixture is used or the
like is identified to some extent. Therefore, in this embodiment 2, the
third frequency f3 as well as the first frequency f1 and the second
frequency f2 is determined by experiment or other means according to the
composition or use environment of the fuel mixture and previously stored
in the controller 12. The fuel-derived capacitance constituent Cf can be
calculated by substituting the third frequency f3 and the resistance
value R3 into the formula (1) described above.

[0069] FIG. 7 is a flowchart for illustrating a control routine according
to the embodiment 2 of the present invention. The routine shown in FIG. 7
differs from the routine shown in FIG. 5 only in that the processing of
Step S16 in FIG. 5 is omitted, and processings of Steps S30 and S32
follow the processing of Step S22.

[0070] Specifically, according to the routine shown in FIG. 7, after the
processing of Step S22, the fuel-derived capacitance constituent Cf is
calculated (S30). More specifically, the fuel-derived capacitance
constituent is calculated by substituting the resistance values R1 and R2
calculated in Step S22 and the third frequency f3 previously stored in
the controller 12 into the formula (1) described above.

[0071] Then, the temperature of the fuel mixture is calculated (S32). The
temperature is calculated based on the map previously stored in the
controller 12 according to the inverse of the fuel resistance constituent
Rf calculated in Step S22 (that is, the conductivity) and the value of
the fuel-derived capacitance constituent Cf.

[0072] Then, the alcohol concentration is determined (S24). In this
example, the alcohol concentration is determined from the temperature
calculated in Step S32 and the fuel resistance constituent Rf.

[0073] As described above, according to the embodiment 2, not only the
alcohol concentration but also the temperature of the fuel mixture can be
detected by one and the same apparatus. Therefore, there is no need for
installing a temperature sensor or the like, so that the cost and size of
the system can be reduced.

[0074] In the embodiment 2, a case has been described where the
fuel-derived capacitance constituent Cf is determined from the third
frequency f3 determined previously and the resistance value R3 determined
approximately from the resistance values R1 and R2. However, the present
invention is not limited to this implementation. For example, the
fuel-derived capacitance constituent Cf may be determined from a complex
impedance curve such as shown in FIG. 4 by changing the frequency a
plurality of number of times.

[0075] In the embodiment 2, to perform Step S30 implements "capacitance
constituent calculating means" according to the present invention, and to
perform Step S32 implements "temperature estimating means" according to
the present invention.

Embodiment 3

[0076] FIG. 8 is a graph for illustrating a variation of an equivalent
concentration value calculated by a concentration detecting apparatus
with respect to a variation of the water content of a fuel mixture
according to an embodiment 3 of the present invention. In this drawing,
the horizontal axis indicates the water content [wt %], and the vertical
axis indicates the equivalent concentration value [wt %]. The lines (a),
(b) and (c) represent cases where the initial concentration of ethanol in
the fuel mixture is 100%, 85% and 22%, respectively.

[0077] The dielectric constant of water is approximately 3.3 times higher
than that of ethanol. Therefore, when the fuel mixture containing
gasoline and ethanol is used, if the water content of the ethanol
increases by 1%, the capacitance increases by 1.5%. Therefore, in the
case shown by the line (b) where the concentration of the ethanol blended
with the gasoline in the fuel is 85%, for example, when the water content
increases by 1%, the detection value shows that the ethanol concentration
has increased by 1.5%.

[0078] In this way, there is a correlation between the water content of
the fuel mixture and the variation of the capacitance. In addition, since
the alcohol concentration of the fuel mixture varies, the conductivity
also varies accordingly. Therefore, there is a particular correlation
between the fuel resistance constituent Rf and the water content.

[0079] Therefore, if the constituents of the fuel mixture are identified,
the alcohol concentration of the fuel mixture can be determined and the
water concentration of the fuel mixture can be determined by using the
fuel-derived capacitance constituent Cf, the fuel resistance constituent
Rf and the temperature detected by the temperature sensor as parameters.
In the embodiment 3, the relationships between the fuel-derived
capacitance constituent Cf, the fuel resistance constituent Rf and the
temperature T and the alcohol concentration and the water concentration
are previously determined by experiment or other means and stored in the
controller 12 in the form of a map. In actual concentration detection,
the fuel-derived capacitance constituent Cf, the fuel resistance
constituent Rf and the temperature T are determined according to the
method described in the embodiment 1 or 2, and the alcohol concentration
and the water concentration are determined based on the map.

[0080] As described above, in the embodiment 3, the water concentration of
the fuel mixture can be detected by detecting the fuel-derived
capacitance constituent Cf and the fuel resistance constituent Rf.
Therefore, both the alcohol concentration and the water concentration can
be detected by one and the same apparatus, and the properties of the fuel
can be more accurately grasped without increasing the size of the
apparatus.